1. Field of the Invention
The present invention relates in general to spark plugs for internal combustion engines and, more particularly, to spark plugs having multiple ground electrodes forming large, three-dimensional spark volumes.
2. Description of the Related Art
As is well known, an internal combustion engine is a type of engine where the expansion of gases produced by combustion applies force to some component of the engine. In a reciprocating engine, the piston moves up and down within a cylinder and transfers force from expanding gas to turn a crankshaft via a connecting rod. The piston is usually made gas-tight with the cylinder using piston rings. The combustion chamber consists of the space within the cylinder above the piston where the burning of the fuel/air mixture occurs.
There are various kinds of internal combustion engines, but the most common variants are two-stroke and four-stroke, gasoline powered engines. Such engines have at least one cylinder, and often have more (e.g., 4, 6, 8, 12 cylinders, etc.). Regardless of the cycle type and number of cylinders, an air-fuel mixture is compressed by the piston when it moves in one direction (i.e., the compression stroke) and then ignited by a spark plug to drive the piston in the opposite direction (i.e., the combustion stroke).
In a two-stroke engine, the piston completes a full power cycle in only two strokes because the end of the combustion stroke and the beginning of the compression stroke happen at the same time, and because the intake and exhaust functions also happen at the same time. This is possible because the reciprocating piston blocks and unblocks intake and exhaust ports that are located in the side wall of the cylinder.
In a four-stroke engine by contrast, commonly used in automotive applications, the piston completes four separate strokes per power cycle, including intake, compression, power, and exhaust strokes. A four-stroke engine typically uses intake and exhaust valves that are located in the cylinder head that seal the piston within the cylinder. The intake and exhaust valves open and close corresponding ports at the appropriate time and for an appropriate duration during the intake and exhaust strokes of each four-stroke power cycle (i.e., intake, compression, power, and exhaust strokes).
The combustion is accomplished by combining a fuel (e.g., gasoline) with an oxidizer (e.g., air) to create a fuel-air mixture and then igniting the fuel-air mixture with an ignition system. In a traditional vehicle, the ignition system consists of several spark plugs (one for each cylinder), an ignition coil or other source of high voltage, a distributor that directs the high voltage from the ignition coil to an output associated with each spark plug, and spark plug wires that carry the high voltage from the outputs of the distributor to each corresponding spark plug and thereby induces a spark that ignites the surrounding fuel-air mixture.
A spark plug ignites the fuel/air mixture in a gasoline engine. According to Wikipedia, a spark plug is “[a] device for delivering electric current from an ignition system to the combustion chamber of a spark-ignition engine to ignite the compressed fuel/air mixture by an electric spark, while containing combustion pressure within the engine.”
FIG. 1 shows a typical J-type or single-electrode spark plug 110. It comprises a metal spark plug shell 120 having with threads 122 that engage a threaded hole in the cylinder head and a single ground electrode 130 that protrudes from a bottom 121 of the spark plug shell 120 and extends downward and then inward to provide the familiar J-shape, an insulated body 140 (e.g., porcelain, high purity alumina, etc.), a center electrode 150 that is surrounded by the insulated body 140 and extends from a terminal 160 that mates with a spark plug wire (not shown) to extend out of the bottom of the insulated body 140 where it terminates very near to the ground electrode 130. The space between the center electrode 150 and the ground electrode 130 defines the “spark gap” 135. If desired, the gap 135 can be adjusted by bending the ground electrode 130 with a suitable tool.
In operation, when high voltage is supplied to the center electrode 150, spaced very near the ground electrode 130, the fuel/air mixture in the spark gap 135 becomes ionized, forming a low resistance electrical path, and the spark plug “fires” by having a spark jump the gap between the two electrodes. The spark ignites the fuel/air mixture located within the combustion chamber, which rapidly burns, expands, and moves the piston within the cylinder.
Engineers have used various techniques to try to create a more homogenous fuel/air mixture that leads to a more efficient engine. For example, in an effort to create and control turbulence, some may have modified the configuration of the combustion chamber by changing the shape of the piston head or internal shape of the cylinder head, or by increasing the number of valves and corresponding ports in an attempt to inject the fuel/air mixture in a spiral pattern, for example. Nonetheless, the fuel/air mixture remains non-homogenous especially at low engine revolutions per minute (“RPMs”), consistent with “stop and go” driving typical of city driving, resulting in imperfect/slow combustion, fouled plugs, increased emissions/pollution, and lower fuel economy. Cars driven on highways at more constant speeds (rather than the “stop and go” type of city driving) keep the engines running above 2000 RPMs and makes the fuel/air mixture more homogenous and hence the cars will have less emissions/pollution and will be more efficient.
The market has seen some multi-electrode spark plugs that offer varying degrees of improvement over a traditional J-type spark plug, but they still suffer from certain deficiencies. For example, FIG. 2 shows an exemplary spark-plug 210 that has two ground electrodes 230 on opposite sides of a center electrode 250. In similar fashion, FIG. 3 shows another exemplary spark plug 310 that has four ground electrodes 330 that surround a center electrode 350.
Some believe that the spark plugs 210 and 310 shown in FIG. 2 and FIG. 3 are not helping more on the “stop and go” type of city driving but are helping for longer mileage between spark plug changes due to the fact that when one ground electrode becomes fouled, another ground electrode will inherently become more attractive to the spark by virtue of it not yet being fouled. However, each individual ground electrode circumferentially offer a limited and very narrow target volume/area for a spark jump, and each ground electrode extends from the spark plug like a conventional J-type electrode such that the extension tends to hinder the spark's access to the adjacent fuel/air mixture.
In addition, the J-type of the electrodes 330 from FIG. 3 and the way how the electrodes 330 are arranged will slow down the propagation of the explosion inside the combustion chamber leading back to slow and inefficient burn of the air fuel mixture, increasing the emissions and lowering the mileage. Further increasing the numbers of J-type electrodes to 5, 6, or more electrodes will shield even more the sparking area from the rest of the combustion chamber slowing down the propagation of the explosion and canceling the benefit of having 2, 3, 4, or more sparking paths.
According, there exists a need for improving the performance of spark plugs.